It's much more simple than that. We live in a world where we are all surrounded by the influence of Spanish-speaking cultures. We've heard of ree-oh day jan-erro. ;) The sailor of the days when that chantey was created would have more often than not been illiterate or barely above such. Someone reading "rio grande" in that context would have no reason to think ree-oh grahn-day, especially since in old English, things like shop could be spelled Shoppe and pronounced with the silent E. Just my two cents.
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